After flying this drone trouble free, I am preparing it to sell and upgrade to another phantom. I flew it yesterday no problem, but today when I attach the battery, after turning on the controller, I get a fast flashing red light and can't start the motors. I have tried different duly charged batteries. Help!!

After flying this drone trouble free, I am preparing it to sell and upgrade to another phantom. I flew it yesterday no problem, but today when I attach the battery, after turning on the controller, I get a fast flashing red light and can't start the motors. I have tried different duly charged batteries. Help!!